Electronic apparatus

ABSTRACT

A first printed wiring board extends in the vertical direction within a first duct that extends in the vertical direction. A first axial flow fan generates airflow which absorbs heat from the first printed wiring board. Second and third ducts extends in parallel with the first duct. A fourth duct extends between the second and third ducts. A second printed wiring board extends in the horizontal direction within the fourth duct. A second axial flow fan is connected to the third duct. The second axial flow fan generates airflow which absorbs heat from the second printed wiring board. The electronic apparatus can be reduced in size.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic apparatus such as aserver computer.

2. Description of the Prior Art

A system board unit is incorporated in a server computer. The systemboard unit includes a printed circuit board unit kept in an uprightattitude. The printed circuit board unit is placed in a duct extendingin the vertical direction. The air outlet of the system board unit isopposed to an axial flow fan rotating around the vertical axis. Airflowruns in the vertical direction in response to the rotation of the axialflow fan.

An electronic component unit serving as a crossbar switch isincorporated in the server computer, for example. The electroniccomponent unit includes a printed circuit board unit kept in ahorizontal attitude. The printed circuit board unit is incorporated in aduct extending in the horizontal direction. The air outlet of theelectronic component unit is opposed to an axial flow fan rotatingaround the horizontal axis. Airflow runs in the horizontal direction inresponse to the rotation of the axial flow fan.

The server computer includes the axial flow fan rotating around thevertical axis and the axial flow fan rotating around the horizontal axisfor cooling the printed circuit board units. These axial flow fansrespectively occupy larger spaces in the vertical and horizontaldirections within the server computer. The server computer inevitablygets larger in size.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide anelectronic apparatus capable of enjoying a reduced size withoutpreventing an efficient airflow therein.

According to a first aspect of the present invention, there is providedan electronic apparatus comprising: an enclosure; a first duct definedin the enclosure, the first duct extending in the vertical direction; afirst printed wiring board or boards extending in the vertical directionwithin the first duct; a first axial flow fan placed within the firstduct, the first axial flow fan including a rotor rotating around thevertical axis; second and third ducts defined in the enclosure inparallel with the first duct; a fourth duct extending in the horizontaldirection at least between the second and third ducts; a second printedwiring board extending in the horizontal direction within the fourthduct; and a second axial flow fan connected at least to the third duct,the second axial flow fan including a rotor rotating around the verticalaxis.

The electronic apparatus allows airflow to run in the vertical directionwithin the first duct with the assistance of the first axial flow fan.The airflow absorbs heat from the first printed wiring board. The firstprinted wiring board is thus cooled down. Airflow is also allowed to runin the horizontal direction within the fourth duct with the assistanceof the second axial flow fan. The airflow absorbs heat from the secondprinted wiring board. The second printed wiring board is thus cooleddown. A superior airflow is realized in the enclosure. Moreover, airflowis only allowed to flow into the fourth duct in the vertical direction.The enclosure or electronic apparatus is thus allowed to have a reducedextend in the horizontal direction as compared with the case where anaxial flow fan having a rotor rotating around a horizontal axis isutilized to introduce airflow into the fourth duct in the horizontaldirection. The electronic apparatus is in this manner allowed to enjoy areduction in size.

The first and second axial flow fans may be arranged along a commonhorizontal plane in the electronic apparatus. The first and second axialflow fans can be placed in a packed manner in a predetermined space inthe enclosure. The second printed wiring board may intersect the firstprinted wiring boards. The second printed wiring board may serve toestablish an electric connection between the first printed wiringboards.

According to a second aspect of the present invention, there is providedan electronic apparatus comprising: an enclosure; first and second ductsdefined in the enclosure, the first and second ducts extending in afirst direction; a printed wiring board or boards extending in a seconddirection perpendicular to the first direction between the first andsecond ducts; and an axial flow fan or fans connected to at least one ofthe first and second ducts, the axial flow fan or fans each having arotor rotating around the rotation axis extending in the firstdirection.

The electronic apparatus allows airflow to run in the first directionwith the assistance of the rotor or rotors of the axial flow fan orfans. The airflow runs in the second direction along the surface orsurfaces of the printed wiring board or boards. The airflow absorbs heatfrom the printed wiring board or boards. The printed wiring board orboards are thus cooled down. In this case, such airflow is directedtoward the surface or surfaces of the printed wiring board or boards inthe first direction. The enclosure or electronic apparatus is thusallowed to have a reduced extend in the horizontal direction as comparedwith the case where an axial flow fan having a rotor rotating around ahorizontal axis is utilized to direct airflow toward the printed wiringboard or boards in the horizontal direction. The electronic apparatus isin this manner allowed to enjoy a reduction in size.

The electronic apparatus may further comprise: a third duct extendingbetween the first and second ducts along a first printed wiring boardselected from the printed wiring boards; and a fourth duct extendingbetween the first and second ducts along a second printed wiring boardselected from the printed wiring boards at a position downstream of thethird duct, the fourth duct having a pressure loss larger than thepressure loss of the third duct.

The larger pressure loss of the fourth duct causes a reduction in theamount of airflow introduced into the fourth duct. This results in anincrease in the amount of airflow introduced into the third duct at aposition upstream of the fourth duct. The amount of airflow can thus beequalized between the third and fourth ducts. A superior airflow can berealized in all the first to fourth ducts.

The fourth duct may be designed to define an air passage at least partlynarrower than an air passage defined in the third duct. When the airpassage defined in the fourth duct is at least partly narrower than theair passage defined in the third duct, the pressure loss of the fourthduct can be set larger than that of the third duct.

Likewise, an opening defined between the fourth and first ducts may benarrower than an opening defined between the third and first ducts. Whenthe opening defined between the fourth and first ducts is narrower thanthe opening defined between the third and first ducts, the pressure lossof the fourth duct can be set larger than that of the third duct.

A specific electronic component unit may be employed to realize theaforementioned electronic apparatus. The electronic component unit maycomprise: a printed wiring board; an electronic component mounted on thesurface of the printed wiring board; a duct lying adjacent to the outerperiphery of the printed wiring board at a position outside the printedwiring board, the duct extending in a direction perpendicular to thesurface of the printed wiring board; and an opening formed in the ductalong the surface of the printed wiring board.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description of thepreferred embodiment in conjunction with the accompanying drawings,wherein:

FIG. 1 is a perspective view from the front side schematicallyillustrating a server computer as a specific example of an electronicapparatus according to an embodiment of the present invention;

FIG. 2 is a perspective view from the rear side schematicallyillustrating the sever computer;

FIG. 3 is a sectional view schematically illustrating the innerstructure of the server computer;

FIG. 4 is a perspective view schematically illustrating the structure ofa first electronic component unit;

FIG. 5 is a perspective view schematically illustrating the structure ofthe first electronic component unit without first and second partitionboards;

FIG. 6 is a perspective view schematically illustrating the structure ofa second electronic component unit;

FIG. 7 is a perspective view schematically illustrating the structure ofthe second electronic component unit;

FIG. 8 is a front plan view schematically illustrating the innerstructure of the server computer;

FIG. 9 is a front plan view schematically illustrating the path ofairflow;

FIG. 10 is a perspective view schematically illustrating the path ofairflow; and

FIG. 11 is a perspective view schematically illustrating the path ofairflow.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates a server computer 11 as a specificexample of an electronic apparatus according to an embodiment of thepresent invention. The server computer 11 includes an enclosure 12.Input/output units 13 are mounted on the lower rack of the enclosure 12at the foreside of the enclosure 12. The input/output units 13 are keptin upright attitudes in the vertical direction perpendicular to thefloor. A PCI board is incorporated in the individual input/output unit13, for example. A LAN cable is coupled to the PCI board, for example.

System board units 14 are mounted on the middle rack of the enclosure12. The system board units 14 are kept in upright attitudes in thevertical direction. A printed wiring board or system board isincorporated in the individual system board unit 14. Electronic circuitelements such as a central processing unit (CPU), a memory, and thelike, are mounted on the system board, for example. The CPU executesvarious kinds of processing based on software programs and/or datatemporarily stored in the memory, for example.

First fan units 15 are mounted on the upper rack of the enclosure 12,for example. The individual first fan unit 15 includes first axial flowfans as described later in detail. The first axial flow fans serve togenerate airflow running in the vertical direction from the bottom ofthe enclosure 12 toward the top of the enclosure 12.

Power source units 16 are mounted on the uppermost rack of the enclosure12. The power source units 16 serve to transform the alternating currentsupplied from an outlet into the direct current. The direct current issupplied to the fan units 15 and a back panel placed within theenclosure 12.

The enclosure 12 includes a panel at the top of the enclosure 12, namelya top panel 12 a, extending in the horizontal direction parallel to thefloor. An air discharge opening 17 is formed in the top panel 12 a. Theenclosure 12 also includes panels at the opposite sides of the enclosure12, namely side panels 12 b, extending in the vertical direction,respectively.

As shown in FIG. 2, input/output units 18 are mounted on the lower rackof the enclosure 12 at the backside of the enclosure 12. Theinput/output units 18 are kept in upright attitudes in the verticaldirection perpendicular to the floor. A PCI board is incorporated in theindividual input/output unit 18 in the same manner as the aforementionedinput/output unit 13. A LAN cable is coupled to the PCI board, forexample.

A pair of first electronic component unit sets 19, 19 are for examplemounted on the middle rack of the enclosure 12 on the backside of theenclosure 12. The individual first electronic component unit set 19includes first electronic component units 21, 21, 21 arranged in threetiers in the vertical direction, for example. The first electroniccomponent units 21 are kept in horizontal attitudes. Description will bemade on the first electronic component units 21 later in detail.

Second electronic component units 22 are mounted on the middle rack ofthe enclosure 12 at the backside of the enclosure 12. The secondelectronic component units 22 are arranged in six tiers in the verticaldirection, for example. The second electronic component units 22 arekept in horizontal attitudes. Description will be made on the secondelectronic component units 22 later in detail.

Second fan units 23,23 are mounted on the upper rack of the enclosure 12on the backside of the enclosure 12. The individual second fan unit 23includes second axial flow fans, for example, as described later indetail. The second axial flow fan units 23 serve to generate airflowrunning in the vertical direction from the bottom of the enclosure 12toward the top of the enclosure 12.

As shown in FIG. 3, an air intake opening 25 is defined in the bottom ofthe enclosure 12. The air intake opening 25 is opposed to the floor. Afresh air is introduced into the inner space of the enclosure 12 throughthe air intake opening 25. The back panel 26 is placed in the innerspace of the enclosure 12. The back panel 26 stands upright in thevertical direction. The input/output units 13 and the system board units14 are coupled to the front surface of the back panel 26. Theinput/output units 18 and the first and second electronic componentunits 21, 22 are coupled to the back surface of the back panel 26.

A duct 27 is defined between the back panel 26 and the front panels ofthe input/output units 13 as well as between the back panel 26 and thefront panels of the system board units 14 in a space between the sidepanels 12 b, 12 b. The duct 27 extends in the vertical direction fromthe air intake opening 25 to the first fan units 15. The first fan units15 are thus placed at the end of the duct 27.

The individual system board unit 14 includes a system board 28 extendingin the vertical direction within the duct 27. The system boards 28 arekept in attitude upright to the back panel 26 in the system board units14, respectively. The system boards 28 are arranged in parallel with oneanother.

The individual first fan unit 15 includes first axial flow fans 29. Theindividual first axial flow fan 29 includes a rotor rotating around thevertical axis perpendicular to the floor. The rotation of the rotorgenerates airflow running in the vertical direction through the duct 27.The airflow absorbs heat from the electronic circuit elements on the PCIboards and the system boards 28. The airflow is discharged through theair discharge opening 17.

The individual second fan unit 23 includes second axial flow fans 31.The first and second axial flow fans 29, 31 are arranged along commonhorizontal planes. The individual second axial flow fan 31 includes arotor rotating around the vertical axis perpendicular to the floor. Therotation of the rotor generates airflow running from the air intakeopening 25 toward the air discharge opening 17.

FIG. 4 schematically illustrates the first electronic component unitsets 19 placed within the server computer 11. As shown in FIG. 4, thefirst electronic component unit sets 19, 19 are placed in a spacebetween first and second partition boards 33, 34 extending in thehorizontal direction. The first partition board 33 supports the firstelectronic component unit sets 19, 19. The second partition board 34covers over the first electronic component unit sets 19, 19. The firstpartition board 33 is opposed to the upper ends of the input/outputunits 13. The second partition board 34 is opposed to the lowest one ofthe second electronic component units 22.

An opening, not shown, is defined in the first partition board 33between the first electronic component unit sets 19, 19. A space extendsin the vertical direction from the opening to the second partition board34. This space functions as a central duct. The central duct is definedbetween the back panel 26 and a wall member 36. The wall member 36extends from the front edge of the first partition board 33 to the frontedge of the second partition board 34 in a space between the firstelectronic component unit sets 19, 19. The central duct extends inparallel with the duct 27 in the enclosure 12.

Air inlets 37 are formed in the first partition board 33 between theouter peripheries of the first electronic component unit sets 19 and thecorresponding side panels 12 b, respectively. Likewise, air outlets 38are formed in the second partition board 34 between the outerperipheries of the first electronic component unit sets 19 and thecorresponding side panels 12 b, respectively. Right and left ducts 39,41 are defined between the first electronic component unit sets 19 andthe corresponding side panels 12 b, respectively. The right and leftducts 39, 41 extend in the vertical direction from the air inlets 37 tothe air outlets 38, respectively. The right and left ducts 39, 41 extendin parallel with the aforementioned duct 27.

The side panels 12 b, the back panel 26 and the wall members 42 incombination serve to define the right and left ducts 39, 41. The wallmembers 42 extend from the front edge of the first partition board 33 tothe front edge of the second partition board 34 in a space between thefirst electronic component unit sets 19 and the corresponding sidepanels 12 b. Here, the right duct 39 has the air inlet 37 and the airoutlet 38 larger than the air inlet 37 and the air outlet 38 of the leftduct 41, respectively. Specifically, the air passage of the right duct39 is set thicker than that of the left duct 41.

The individual first electronic component unit 21 includes a printedwiring board 43 in the shape of a rectangle extending in the horizontaldirection. The rear edge of the printed wiring board 43 is coupled tothe back panel 26 at one of the sides of the rectangle. A front panel 44stands upright in the vertical direction from the printed wiring board43 at the front edge of the printed wiring board 43. A horizontal duct45 is defined between the front panel 44 and the back panel 26 in aspace between the adjacent ones of the printed wiring boards 43. Itshould be noted that the uppermost one of the horizontal ducts 45 isdefined between the printed wiring board 43 and the second partitionboard 34.

Referring also to FIG. 5, the horizontal ducts 45 extend in thehorizontal direction between the central duct 46 and the right duct 39as well as between the central duct 46 and the left duct 41. Thehorizontal ducts 45 serve to connect the central duct 46 to the rightand left ducts 39, 41, respectively. Electronic components such aslarge-scale integrated circuit (LSI) packages 47, a memory, and thelike, are mounted on the individual printed wiring board 43. A centralprocessing unit (CPU) chip is incorporated in the individual LSI package47, for example. The CPU executes various kinds of processing based onsoftware programs and/or data temporarily stored in the memory, forexample. A cooling fin may be mounted on the individual electroniccomponent, for example. The fin may extend in parallel with the backpanel 26 in an upright attitude, for example.

The first electronic component units 21 are utilized for systemmanagement of the server computer 11, for example. A LAN cable iscoupled to the individual first electronic component unit 21, forexample. A personal computer can be coupled to the server computer 11through the LAN cable, for example.

As shown in FIG. 6, the second electronic component units 22 are placedin a space between third and fourth partition boards 51, 52. The thirdpartition board 51 supports the lowest one of the second electroniccomponent units 22. The fourth partition board 52 covers over theuppermost one of the second electronic component units 22. The thirdpartition board 51 is opposed to the uppermost one of the firstelectronic component units 21. The fourth partition board 52 is opposedto the lowest one of the second fan units 23.

Air inlets 53 are formed in the third partition board 51 between theouter periphery of the lowest one of the second electronic componentunits 22 and the side panels 12 b. A right duct 54 and a left duct aredefined between the second electronic component units 22 and the sidepanels 12 b, respectively. The right duct 54 and the left duct extend inthe vertical direction from the air inlets 53, respectively. The rightduct 54 and the left duct extend in parallel with the aforementionedduct 27. The side panels 12 b, the back panel 26, wall members 55 andthe fourth partition board 52 in combination serve to define the rightduct 54 and the left duct, respectively. The wall members 55 extend fromthe front edge of the third partition board 51 to the front edge of thefourth partition board 52 in a space between the second electroniccomponent units 22 and the side panels 12 b.

The individual second electronic component unit 22 includes a printedwiring board 56 in the shape of a rectangle extending in the horizontaldirection. The rear edge of the printed wiring board 56 is coupled tothe back panel 26 at one of the sides of the rectangle. A duct 57extends in the vertical direction along the front edge of the printedwiring board 56 in a space outside the printed wiring board 56. Theuppermost one of the second electronic component units 22 allows theupper end of the duct 57 to be coupled to an air outlet 60 formed in thefourth partition board 52. A rear panel 58 stands upright in thevertical direction from the printed wiring board 56 at the rear edge ofthe printed wiring board 56.

The duct 57 extends in the lateral direction of the second electroniccomponent unit 22 along the front edge of the printed wiring board 56.An opening 59 is formed in the duct 57 along the surface of the printedwiring board 56. The ducts 57, 57 of the adjacent ones of the secondelectronic component units 22 are coupled to each other. The ducts 57 ofthe second electronic component units 22 are thus arranged in six tiersin the vertical direction. This results in establishment of a verticalduct 61 extending in the vertical direction.

A horizontal duct 62 is defined between the duct 57 and the rear panel58 in a space between the adjacent ones of the printed wiring boards 56.The uppermost one of the horizontal ducts 62 is defined between theprinted wiring board 56 and the fourth partition board 52. Referringalso to FIG. 7, the horizontal ducts 62 extend in the horizontaldirection between the right duct 54 and the vertical duct 61 as well asbetween the left duct 63 and the vertical duct 61.

LSI packages 64, 64 are mounted on the individual printed wiring board56, for example. A controller chip is embedded in the individual LSIpackage 64, for example. The controller chip functions as a crossbarswitch for data transferred between the CPUs on the system boards 28 orbetween the CPUs and the input/output units 13, for example. The systemboards 28 in the upright attitudes are arranged in the horizontaldirection along the surface of the back panel 26 as described above. Theprinted wiring board 56 in the individual second electronic componentunit 22 thus intersects all the system boards 28 along the back surfaceof the back panel 26. The individual printed wiring board 56 serves toestablish an electric connection between the system boards 28.

Here, the pressure loss of the horizontal duct 62 located at adownstream position in the right and left ducts 54, 63 is set largerthan that of the horizontal duct 62 located at an upstream position. Inthis case, the cross section of the horizontal duct 62 at a downstreamposition or an upper position may be narrower than that of thehorizontal duct 62 at an upstream position or a lower position.Alternatively, the opening 59 of the horizontal duct 62 at an upperposition may be narrower than the opening 59 of the horizontal duct 62at a lower position.

Such an increase in the pressure loss in the downstream directionresults in a reduction in the amount of airflow running through thehorizontal duct 62 at a downstream position. The horizontal duct 62 atan upstream position is correspondingly allowed to enjoy an increase inthe amount of airflow. In this manner, all the horizontal ducts 62 areallowed to enjoy a sufficient amount of airflow. The airflow of asufficient amount runs through all of the horizontal ducts 62. Thesecond electronic component units 22 can evenly be cooled.

As shown in FIG. 8, the upper ends of the input/output units 18 areconnected to the central duct 46, the right duct 39 and the left duct41. The central duct 46, the right duct 39 and the left duct 41 areconnected to the horizontal ducts 45. The second partition board 34serves to define the closed upper end of the central duct 46. The rightduct 39 is connected to the right duct 54. The left duct 41 is connectedto the left duct 63.

The vertical duct 61 is connected to the second axial flow fans 31 atthe upper end of the vertical duct 61. The fourth partition board 52serves to define the closed upper end of the right duct 54. The fourthpartition board 52 likewise serves to define the closed upper end of theleft duct 63. The horizontal ducts 62 serve to connect the right duct 54and the left duct 63 to the vertical duct 61.

Now, assume that the second axial flow fans 31 of the individual secondfan unit 23 generate airflow in the enclosure 12. The rotation of therotor of the individual second axial flow fan 31 allows a fresh air toenter the air intake opening 25. As shown in FIG. 9, the airflow runs inthe vertical direction within the input/output units 13. The airflowabsorbs heat from the PCI boards, for example. The PCI boards are thusprevented from a rise in temperature.

Referring also to FIG. 10, the airflow is allowed to flow into thecentral duct 46 through the opening of the first partition board 33.Since the second partition board 34 closes the upper end of the centralduct 46, the airflow is allowed to run into the horizontal ducts 45 fromthe central duct 46. The airflow thus runs along the surfaces of theprinted wiring boards 43. The airflow absorbs heat from the LSI packages47. The CPU chips are thus prevented from a rise in temperature.

Likewise, the airflow is allowed to flow into the right and left ducts39, 41 through the air inlets 37, 37 of the first partition board 33.The airflow is allowed to run into the right and left ducts 39, 41 fromthe horizontal ducts 42. The airflow runs in the vertical direction fromthe air inlets 37 to the air outlets 38 in the right and left ducts 39,41. The airflow is discharged through the air outlets 38 of the secondpartition board 34.

The airflow is then allowed to flow into the right and left ducts 54,63. The airflow runs in the vertical direction within the right and leftducts 54, 63. Since the fourth partition board 52 closes the upper endsof the right and left ducts 54, 63, the airflow is allowed to run intothe horizontal ducts 62 from the right and left ducts 54, 63. Referringalso to FIG. 11, the airflow runs along the surfaces of the printedwiring boards 56. The airflow absorbs heat from the LSI packages 64. Thecontroller chips are thus prevented from a rise in temperature.

The airflow running in the horizontal ducts 62 is then allowed to flowinto the ducts 57, namely the vertical duct 61, through the openings 59.Since the upper end of the vertical duct 61 is connected to the secondaxial flow fans 31, the airflow runs in the vertical direction withinthe vertical duct 61. The airflow thus runs from the vertical duct 61 tothe second axial flow fans 31. The airflow is then discharged throughthe air discharge opening 17.

The server computer 11 allows the system boards 28 to establish theupright or vertical attitudes within the duct 27 extending in thevertical direction. The first axial flow fans 29 generate airflowrunning in the vertical direction within the duct 27. The airflowabsorbs heat from the system boards 28 within the duct 27. The printedwiring boards 43, 56 take the horizontal attitudes within the horizontalducts 45, 62. The horizontal ducts 45, 62 are connected to the centralduct 46, the right and left ducts 39, 41, the right and left ducts 54,63, the vertical ducts 61, all extending in parallel with the duct 27.The vertical duct 61 is connected to the second axial flow fans 31. Thesecond axial flow fans generate airflow running in the verticaldirection. The airflow absorbs heat from the printed wiring boards 43,56 within the horizontal ducts 45, 62. The airflow is allowed toefficiently run within the enclosure 12 in this manner.

The airflow runs in the vertical direction when the airflow is guidedinto the horizontal ducts 45, 62. The enclosure 11 or the servercomputer 11 is allowed to have a reduced extent in the horizontaldirection as compared with the case where axial flow fans having rotorsrotating around the horizontal axes serve to generate horizontal airflowin the horizontal ducts 45, 62. The server computer 11 is in this mannerallowed to enjoy a reduced size. In addition, the first and second axialflow fans 29, 31 can be arranged along a specific horizontal plane in apacked manner in the enclosure 12. This results in simplification of thewiring structure of the first and second axial flow fans 29, 31, forexample.

1. An electronic apparatus comprising: an enclosure; a first ductdefined in the enclosure, said first duct extending in a verticaldirection; a first printed wiring board or boards extending in thevertical direction within the first duct; a first axial flow fanincluding a rotor rotating around a vertical axis; second and thirdducts defined in the enclosure in parallel with the first duct; a fourthduct extending in a horizontal direction at least between the second andthird ducts; a second printed wiring board extending in the horizontaldirection within the fourth duct; and a second axial flow fan connectedat least to the third duct, said second axial flow fan including a rotorrotating around a vertical axis.
 2. The electronic apparatus accordingto claim 1, wherein the first and second axial flow fans are arrangedalong a common horizontal plane.
 3. The electronic apparatus accordingto claim 1, wherein the second printed wiring board intersects the firstprinted wiring boards.
 4. An electronic apparatus comprising: anenclosure; first and second ducts defined in the enclosure, said firstand second ducts extending in a first direction; a printed wiring boardor boards extending in a second direction perpendicular to the firstdirection between the first and second ducts; and an axial flow fan orfans connected to at least one of the first and second ducts, said axialflow fan or fans each having a rotor rotating around a rotation axisextending in the first direction.
 5. The electronic apparatus accordingto claim 4, further comprising: a third duct extending between the firstand second ducts along a first printed wiring board selected from theprinted wiring boards; and a fourth duct extending between the first andsecond ducts along a second printed wiring board selected from theprinted wiring boards at a position downstream of the third duct, saidfourth duct having a pressure loss larger than a pressure loss of thethird duct.
 6. The electronic apparatus according to claim 5, whereinthe fourth duct is designed to define an air passage at least partlynarrower than an air passage defined in the third duct.
 7. Theelectronic apparatus according to claim 5, wherein an opening is definedbetween the fourth and first ducts, said opening being narrower than anopening defined between the third and first ducts.
 8. An electroniccomponent unit comprising: a printed wiring board; an electroniccomponent mounted on a surface of the printed wiring board; a duct lyingadjacent to an outer periphery of the printed wiring board at a positionoutside the printed wiring board, said duct extending in a directionperpendicular to the surface of the printed wiring board; and an openingformed in the duct along the surface of the printed wiring board.